An implantable passive artificial muscle system will be developed and evaluated in vivo. Materials will be mechanically stressed in vivo to determine their ability to function as an artificial muscle for long periods of time. The tendons of the artificial muscle system will be attached to bone and natural tendons. Various attachment schemes will be evaluated during the study. Several different artificial muscle systems will be fabricated and implanted in the legs of sheep. The natural tibialis anterior muscle will be replaced by the passive artificial muscle. The passive muscle will allow the foot to be held in its normal position. Control of the sheep's hoof will be possible since the antagonistic muscle will remain intact. The artificial muscles will be fabricated from rubbers, resins, plastics, metals, and ceramics. Each of the materials used in the muscle system will undergo static and dynamic mechanical stress in vivo. The muscle component materials will be carefully analyzed for degradation and their ability to function under known mechanical stress.